One of the UK’s biggest trade unions is facing potential strike action from some of its own staff in a dispute over pay and home working, a threat that the union’s general secretary has condemned as unnecessary and self-indulgent.

In a bitter internal wrangle at one of the unions most strongly supportive of the Labour leader, Keir Starmer, office staff at the Salford headquarters of Usdaw, which represents retail workers, have rejected a pay offer from the union of 3.5%.

The Usdaw employees, who are represented by the GMB union, are also seeking a commitment over potential home working, which staff say is not permitted.

But Paddy Lillis, the general secretary of Usdaw, the full name of which is the Union of Shop, Distributive and Allied Workers, said the full value of the pay offer was above 11% if an extra £1.3m committed to the union’s final salary pension scheme was taken into account.

Lillis said the dispute involved fewer than 100 of the union’s 400 staff, and that those represented by two other internal organisations had agreed to the union’s pay offer.

Lillis said he was “absolutely livid” with the GMB organisers involved, arguing that the discussions over home working should have been separated from those about pay.

“You’re asking me to change your contract of employment so some of you can work from home,” he said. “To me it’s an issue of policy, and I’ve asked them to not put it on the wage claim, so we can look at it next year.

“My main priority is to get this union back firing on all cylinders. We’ve lost 70,000 members over the two years of the pandemic. Our members are low paid, and all had to go to work during the pandemic to keep the country moving. I’ve told them – pick your battles, and this isn’t one to pick.

“Working from home is not a simple matter. There’s all sorts of issues around health and safety, insurance and keeping the service to members. That’s why I said I’d look at it next year. I’m disappointed in the small number that are holding the rest to ransom. We can’t pay the increase until this is resolved.”

Those involved in the dispute, Lillis added, should consider the plight of many of the shopworkers represented by Usdaw: “Some of them can’t even afford to buy food in their own supermarket, and we’re going on strike when we’re all well paid with good terms and conditions, as you’d expect from a trade union. I’m absolutely livid with them.”

Last month, Lillis called for “a degree of silence” from other union heads who have criticised Starmer over what they said was his lack of support for striking workers.

Karen Lewis, an organiser for the GMB, said: “GMB members employed at Usdaw are in dispute. They are seeking a cost of living pay increase and have unanimously rejected an offer of 3.5%.

“Their claim is to reflect the growing financial insecurities we all face. Staff are also seeking a commitment to explore new ways of working. GMB remains open to finding a mutual resolution to the dispute.”